DNA structure replication transcription translation

The Molecular Basis of Inheritance

Figure 16.7a, c

TA0.34 nm

3.4 nm

(a) Key features of DNA structure

(c) Space-filling model

1962: Nobel Prize in Physiology and Medicine

James D.Watson

Francis H.Crick

Maurice H. F.Wilkins

What about?Rosalind Franklin

Watson, J.D. and F.H. Crick, “Molecular Structure of Nucleic Acids: A Structure for Deoxynucleic Acids”. Nature 171 (1953), p. 738.

The Structure of DNA• DNA is composed of four nucleotides,

each containing: adenine, cytosine, thymine, or guanine.

• The amounts of A = T, G = C, and purines = pyrimidines [Chargaff’s Rule].

• DNA is a double-stranded helix with antiparallel strands [Watson and Crick].

• Nucleotides in each strand are linked by 5’-3’ phosphodiester bonds

• Bases on opposite strands are linked by hydrogen bonding: A with T, and G with C.

The Basic Principle: Base Pairing to a Template Strand

• The relationship between structure and function is manifest in the double helix

• Since the two strands of DNA are complementary each strand acts as a template for building a new strand in replication

DNA replication• The parent molecule unwinds, and two new

daughter strands are built based on base-pairing rules

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(b) The first step in replication is separation of the two DNA strands.

(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

(d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

DNA Replication is “Semi-conservative”

• Each 2-stranded daughter molecule is only half new

• One original strand was used as a template to make the new strand

DNA Replication• The copying of DNA is remarkable in its speed and accuracy• Involves unwinding the double helix and synthesizing two

new strands.• More than a dozen enzymes and other proteins participate

in DNA replication• The replication of a DNA molecule begins at special sites

called origins of replication, where the two strands are separated

Origins of Replication• A eukaryotic chromosome may have hundreds or

even thousands of replication origins

Replication begins at specific siteswhere the two parental strandsseparate and form replicationbubbles.

The bubbles expand laterally, asDNA replication proceeds in bothdirections.

Eventually, the replicationbubbles fuse, and synthesis ofthe daughter strands iscomplete.

Origin of replication

Bubble

Parental (template) strandDaughter (new) strand

Replication fork

Two daughter DNA molecules

In eukaryotes, DNA replication begins at many sites along the giantDNA molecule of each chromosome.

In this micrograph, three replicationbubbles are visible along the DNA ofa cultured Chinese hamster cell (TEM).

(b)(a)

0.25 µm

Mechanism of DNA Replication• DNA replication is catalyzed by DNA polymerase which needs an RNA primer• RNA primase synthesizes primer on DNA strand• DNA polymerase adds nucleotides to the 3’ end of the growing strand

Mechanism of DNA Replication• Nucleotides are added by complementary base pairing with the

template strand• The substrates, deoxyribonucleoside triphosphates, are

hydrolyzed as added, releasing energy for DNA synthesis.

The Mechanism of DNA Replication• DNA synthesis on the leading strand is continuous • The lagging strand grows the same general direction as the leading

strand (in the same direction as the Replication Fork). However, DNA is made in the 5’-to-3’ direction

• Therefore, DNA synthesis on the lagging strand is discontinuous• DNA is added as short fragments (Okasaki fragments) that are

subsequently ligated together

DNA polymerase I degrades the RNA primer and replaces it with DNA

The Mechanism of DNA Replication• Many proteins assist in DNA replication

• DNA helicases unwind the double helix, the template strands are stabilized by other proteins

• Single-stranded DNA binding proteins make the template available

• RNA primase catalyzes the synthesis of short RNA primers, to which nucleotides are added.

• DNA polymerase III extends the strand in the 5’-to-3’ direction

• DNA polymerase I degrades the RNA primer and replaces it with DNA

• DNA ligase joins the DNA fragments into a continuous daughter strand

Enzymes in DNA replication

Helicase unwinds parental double helix

Binding proteinsstabilize separatestrands

DNA polymerase III binds nucleotides to form new strands

Ligase joins Okazaki fragments and seals other nicks in sugar-phosphate backbone

Primase adds short primer to template strand

DNA polymerase I (Exonuclease) removes RNA primer and inserts the correct bases

Binding proteins prevent single strands from rewinding.

Helicase protein binds to DNA sequences called origins and unwinds DNA strands.

5’ 3’

Primase protein makes a short segment of RNA complementary to the DNA, a primer.

3’ 5’

5’ 3’

Replication

Overall directionof replication

5’ 3’5’

3’ 5’

DNA polymerase enzyme adds DNA nucleotides to the RNA primer.

Replication

DNA polymerase enzyme adds DNA nucleotides to the RNA primer.

Overall directionof replication

DNA polymerase proofreads bases added and replaces incorrect nucleotides.

Replication

5’ 3’

3’ 5’

3’Overall directionof replication

Leading strand synthesis continues in a 5’ to 3’ direction.

Replication

3’ 5’ 5’

5’ 3’

3’ 5’

Okazaki fragment

Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.

Replication

5’ 5’

5’ 3’

3’ 5’

Okazaki fragment

Replication

5’ 3’

5’ 5’ 3’

Replication

3’ 5’

5’ 3’

5’ 5’ 3’

Replication

3’ 3’

5’ 3’

Exonuclease activity of DNA polymerase I removes RNA primers.

Replication

Polymerase activity of DNA polymerase I fills the gaps.

Ligase forms bonds between sugar-phosphate backbone.

5’ 3’

Replication

Replication Fork Overview

Proofreading• DNA must be faithfully replicated…but

mistakes occur– DNA polymerase (DNA pol) inserts the wrong

nucleotide base in 1/10,000 bases• DNA pol has a proofreading capability and can correct

errors– Mismatch repair: ‘wrong’ inserted base can be

removed– Excision repair: DNA may be damaged by

chemicals, radiation, etc. Mechanism to cut out and replace with correct bases

Mutations

• A mismatching of base pairs, can occur at a rate of 1 per 10,000 bases.

• DNA polymerase proofreads and repairs accidental mismatched pairs.

• Chances of a mutation occurring at any one gene is over 1 in 100,000

• Because the human genome is so large, even at this rate, mutations add up. Each of us probably inherited 3-4 mutations!

Proofreading and Repairing DNA

• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides

• In mismatch repair of DNA, repair enzymes correct errors in base pairing

• In nucleotide excision DNA repair nucleases cut out and replace damaged stretches of DNA

Nuclease

DNApolymerase

DNAligase

A thymine dimerdistorts the DNA molecule.1

A nuclease enzyme cutsthe damaged DNA strandat two points and thedamaged section isremoved.

Repair synthesis bya DNA polymerasefills in the missingnucleotides.

DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.

Accuracy of DNA Replication

• The chromosome of E. coli bacteria contains about 5 million bases pairs– Capable of copying this DNA in less than an hour

• The 46 chromosomes of a human cell contain about 6 BILLION base pairs of DNA!!– Printed one letter (A,C,T,G) at a time…would fill

up over 900 volumes of Campbell.

– Takes a cell a few hours to copy this DNA

– With amazing accuracy – an average of 1 per billion nucleotides

The Central Dogma of Life.

replication

Protein Synthesis• The information content of DNA is in the form

of specific sequences of nucleotides along the DNA strands

• The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins

• The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation

Transcription and Translation

• Cells are governed by a cellular chain of command– DNA RNA protein

• Transcription– Is the synthesis of RNA under the direction of DNA– Produces messenger RNA (mRNA)

• Translation– Is the actual synthesis of a polypeptide, which

occurs under the direction of mRNA– Occurs on ribosomes

Transcription and Translation

• In prokaryotes transcription and translation occur together

Figure 17.3a

Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.

TRANSLATION

TRANSCRIPTION DNA

Ribosome

Polypeptide

Transcription and Translation• In a eukaryotic cell the nuclear envelope separates

transcription from translation• Extensive RNA processing occurs in the nucleus

Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

Pre-mRNA

Polypeptide

Ribosome

Nuclearenvelope

Transcription• Transcription is the DNA-

directed synthesis of RNA• RNA synthesis

– Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides

– Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine

Table 17.1

• RNA is single stranded, not double stranded like DNA• RNA is short, only 1 gene long, where DNA is very long and

contains many genes• RNA uses the sugar ribose instead of deoxyribose in DNA• RNA uses the base uracil (U) instead of thymine (T) in DNA.

Synthesis of an RNA Transcript

• The stages of transcription are– Initiation– Elongation– Termination

PromoterTranscription unit

RNA polymerase

Start point

Rewound

transcript

3Completed RNA transcript

Unwound

transcript

Template strand of DNA

1 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand.

2 Elongation. The polymerase moves downstream, unwinding theDNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix.

3 Termination. Eventually, the RNAtranscript is released, and the polymerase detaches from the DNA.

• Promoters signal the initiation of RNA synthesis

• Transcription factors help eukaryotic RNA polymerase recognize promoter sequences

• A crucial promoter DNA sequence is called a TATA box.

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

Pre-mRNA

Ribosome

Polypeptide

T A T AAA AATAT T T T

TATA box Start point TemplateDNA strand

Transcriptionfactors

Promoter

RNA polymerase IITranscription factors

RNA transcript

Transcription initiation complex

Eukaryotic promoters1

Several transcriptionfactors

Additional transcriptionfactors

Synthesis of an RNA Transcript - Initiation

Synthesis of an RNA Transcript - Elongation

Elongation

RNApolymerase

Non-templatestrand of DNA

RNA nucleotides

C A E G C A AU

T A G G T TA

T C C A A TT

Newly madeRNA

Direction of transcription(“downstream”) Template

strand of DNA

• RNA polymerase synthesizes a single strand of RNA against the DNA template strand (anti-sense strand), adding nucleotides to the 3’ end of the RNA chain

• As RNA polymerase moves along the DNA it continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides

• Specific sequences in the DNA signal termination of transcription

• When one of these is encountered by the polymerase, the RNA transcript is released from the DNA and the double helix can zip up again.

Synthesis of an RNA Transcript - Termination

Transcription Overview

• Most eukaryotic mRNAs aren’t ready to be translated into protein directly after being transcribed from DNA. mRNA requires processing.

• Transcription of RNA processing occur in the nucleus. After this, the messenger RNA moves to the cytoplasm for translation.

• The cell adds a protective cap to one end, and a tail of A’s to the other end. These both function to protect the RNA from enzymes that would degrade

• Most of the genome consists of non-coding regions called introns

– Non-coding regions may have specific chromosomal functions or have regulatory purposes

– Introns also allow for alternative RNA splicing

• Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things:

– Add protective bases to the ends

– Cut out the introns

Post Termination RNA Processing

Alteration of mRNA Ends• Each end of a pre-mRNA molecule is modified

in a particular way– The 5 end receives a modified nucleotide cap– The 3 end gets a poly-A tail

A modified guanine nucleotideadded to the 5 end

50 to 250 adenine nucleotidesadded to the 3 end

Protein-coding segment Polyadenylation signal

Poly-A tail3 UTRStop codonStart codon

5 Cap 5 UTR

AAUAAA AAA…AAA

TRANSCRIPTION

RNA PROCESSING

Pre-mRNA

TRANSLATIONRibosome

Polypeptide

G P P P

RNA Processing - Splicing

• The original transcript from the DNA is called pre-mRNA.

• It contains transcripts of both introns and exons.

• The introns are removed by a process called splicing to produce messenger RNA (mRNA)

RNA Processing - Splicing

• Ribozymes are catalytic RNA molecules that function as enzymes and can splice RNA

• RNA splicing removes introns and joins exons

Figure 17.10

TRANSCRIPTION

RNA PROCESSING

Pre-mRNA

TRANSLATION

Ribosome

Polypeptide

Exon Intron

104 105 146

Exon 3Poly-A tail

Poly-A tail

Introns cut out andexons spliced together

Codingsegment

5 Cap1 146

3 UTR3 UTR

Pre-mRNA

RNA Processing• RNA Splicing can also be carried out by spliceosomes

RNA transcript (pre-mRNA)

Exon 1 Intron Exon 2

Other proteinsProteinsnRNA

snRNPs

Spliceosome

Spliceosomecomponents

Cut-outintronmRNA

Exon 1 Exon 2

Alternative Splicing (of Exons)• How is it possible that there are millions of human

antibodies when there are only about 30,000 genes?

• Alternative splicing refers to the different ways the exons of a gene may be combined, producing different forms of proteins within the same gene-coding region

• Alternative pre-mRNA splicing is an important mechanism for regulating gene expression in higher eukaryotes

RNA Processing

• Proteins often have a modular architecture consisting of discrete structural and functional regions called domains

• In many cases different exons code for the different domains in a protein

Figure 17.12

GeneDNA

Exon 1 Intron Exon 2 Intron Exon 3

TranscriptionRNA processing

Translation

Domain 3

Domain 1

Domain 2

Polypeptide

Translation

• Translation is the RNA-directed synthesis of a polypeptide

• Translation involves – mRNA– Ribosomes - Ribosomal RNA– Transfer RNA – Genetic coding - codons

TRANSCRIPTION

TRANSLATION

mRNARibosome

Polypeptide

PolypeptideAminoacids

tRNA withamino acidattachedRibosome

Anticodon

Phe Gly

U G G U U U G G C

Codons5 3

The Genetic Code• Genetic information is encoded as a sequence of nonoverlapping

base triplets, or codons• The gene determines the sequence of bases along the length of an

mRNA molecule

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

Protein

TRANSLATION

Amino acid

A C C A A A C C G A G T

U G G U U U G G C U C A

Trp Phe Gly Ser

The Genetic Code• Codons: 3 base code for the production of a specific amino acid,

sequence of three of the four different nucleotides

• Since there are 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons

• 64 codons but only 20 amino acids, therefore most have more than 1 codon

• 3 of the 64 codons are used as STOP signals; they are found at the end of every gene and mark the end of the protein

• One codon is used as a START signal: it is at the start of every protein

• Universal: in all living organisms

The Genetic Code• A codon in messenger RNA is either translated into an

amino acid or serves as a translational start/stop signal

Second mRNA baseU C A G

UUUUUCUUAUUG

CUUCUCCUACUG

AUUAUCAUAAUG

GUUGUCGUAGUG

Met orstart

UCUUCCUCAUCG

CCUCCCCCACCG

ACUACCACAACG

GCUGCCGCAGCG

UAUUAC

UGUUGCTyr Cys

CAUCACCAACAG

CGUCGCCGACGG

AAUAACAAAAAG

AGUAGCAGAAGG

GAUGACGAAGAG

GGUGGCGGAGGG

UGGUAAUAG Stop

Stop UGA StopTrp

UCAGUCAGUCAGUCAG

Transfer RNA• Consists of a single RNA strand that is only about 80

nucleotides long• Each carries a specific amino acid on one end and has an

anticodon on the other end• A special group of enzymes pairs up the proper tRNA molecules

with their corresponding amino acids.• tRNA brings the amino acids to the ribosomes,

Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)

CCACGCUUAAGACACCU*

GC * *

G U G U *CU* G AG

*A A GUC

AGACC*

C G A G A G GG*

*GACUC*AUUUAGGCG5

Amino acidattachment site

Hydrogenbonds

Anticodon

The “anticodon” is the 3 RNA bases that matches the 3 bases of the codon on the mRNA molecule

Transfer RNA• 3 dimensional tRNA molecule is roughly “L” shaped

(b) Three-dimensional structureSymbol used in the book

Amino acidattachment site

Hydrogen bonds

AnticodonAnticodon

Ribosomes• Ribosomes facilitate the specific coupling of tRNA anticodons

with mRNA codons during protein synthesis• The 2 ribosomal subunits are constructed of proteins and RNA

molecules named ribosomal RNA or rRNA

TRANSCRIPTION

TRANSLATION

mRNARibosome

Polypeptide Exit tunnelGrowingpolypeptide

tRNAmolecules

Largesubunit

Smallsubunit

Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins.

Ribosome• The ribosome has three binding sites for tRNA

– The P site– The A site– The E site

P site (Peptidyl-tRNAbinding site)

E site (Exit site)

mRNAbinding site

A site (Aminoacyl-tRNA binding site)

Largesubunit

Smallsubunit

Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.

Amino end Growing polypeptide

Next amino acidto be added topolypeptide chain

Codons

Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site.

Building a Polypeptide

Building a Molecule of tRNA• A specific enzyme called an aminoacyl-tRNA

synthetase joins each amino acid to the correct tRNA

Figure 17.15

Amino acid

Adenosine

Pyrophosphate

Adenosine

Phosphates

Aminoacyl tRNA(an “activatedamino acid”)

Aminoacyl-tRNAsynthetase (enzyme)

Active site binds theamino acid and ATP. 1

ATP loses two P groupsand joins amino acid as AMP.2

3 AppropriatetRNA covalentlyBonds to aminoAcid, displacingAMP.

Activated amino acidis released by the enzyme.4

Building a Polypeptide

• We can divide translation into three stages– Initiation– Elongation– Termination

• The AUG start codon is recognized by methionyl-tRNA or Met

• Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain

• RNA is decoded by tRNA (transfer RNA) molecules, which each transport specific amino acids to the growing chain

• Translation ends when a stop codon (UAA, UAG, UGA) is reached

Initiation of Translation• The initiation stage of translation brings together

mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome

Largeribosomalsubunit

The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiationfactors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid.

Initiator tRNA

mRNA binding site Smallribosomalsubunit

Translation initiation complex

P site

GDPGTP

Start codon

A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met).

Met MetU A CA U G

Elongation of the Polypeptide Chain• In the elongation stage, amino acids are added one

by one to the preceding amino acid

Amino endof polypeptide

Ribosome ready fornext aminoacyl tRNA

GDPGTP

site site5

TRANSCRIPTION

TRANSLATION

mRNARibosome

Polypeptide

Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysisof GTP increases the accuracy andefficiency of this step.

Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site.

Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs,bringing the next codon to be translated into the A site.

Termination of Translation• The final step in translation is termination. When the ribosome reaches a STOP codon, there is no

corresponding transfer RNA. • Instead, a small protein called a “release factor” attaches to the stop codon.• The release factor causes the whole complex to fall apart: messenger RNA, the two ribosome subunits, the new

polypeptide.• The messenger RNA can be translated many times, to produce many protein copies.

Release factor

Freepolypeptide

Stop codon(UAG, UAA, or UGA)

When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

1 The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome.

2 3 The two ribosomal subunits and the other components of the assembly dissociate.

Translation: Initiation • mRNA binds to a ribosome, and the transfer RNA

corresponding to the START codon binds to this complex. Ribosomes are composed of 2 subunits (large and small), which come together when the messenger RNA attaches during the initiation process.

Translation: Elongation• Elongation: the ribosome moves down the messenger RNA,

adding new amino acids to the growing polypeptide chain. • The ribosome has 2 sites for binding transfer RNA. The first

RNA with its attached amino acid binds to the first site, and then the transfer RNA corresponding to the second codon bind to the second site.

Translation: Elongation• The ribosome then removes the amino acid from the

first transfer RNA and attaches it to the second amino acid.

• At this point, the first transfer RNA is empty: no attached amino acid, and the second transfer RNA has a chain of 2 amino acids attached to it.

• The elongation cycle repeats as the ribosome moves down the messenger RNA, translating it one codon and one amino acid at a time.

• The process repeats until a STOP codon is reached.

Translation: Termination

Polyribosomes• A number of ribosomes can translate a single mRNA

molecule simultaneously forming a polyribosome• Polyribosomes enable a cell to make many copies of

a polypeptide very quickly

Growingpolypeptides

Completedpolypeptide

Incomingribosomalsubunits

Start of mRNA(5 end)

End of mRNA(3 end)

Polyribosome

An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes.

RibosomesmRNA

This micrograph shows a large polyribosome in a prokaryotic cell (TEM).

0.1 µm

Comparing Gene Expression In Prokaryotes And Eukaryotes• In a eukaryotic cell:

– The nuclear envelope separates transcription from translation– Extensive RNA processing occurs in the nucleus

• Prokaryotic cells lack a nuclear envelope, allowing translation to begin while transcription progresses

RNA polymerase

Polyribosome

RNApolymerase

Direction oftranscription

0.25 m

Polyribosome

Polypeptide(amino end)

Ribosome

mRNA (5 end)

A summary of transcription and translation in a eukaryotic cell

Figure 17.26

TRANSCRIPTION

RNA is transcribedfrom a DNA template.

RNApolymerase

RNAtranscript

RNA PROCESSING

In eukaryotes, theRNA transcript (pre-mRNA) is spliced andmodified to producemRNA, which movesfrom the nucleus to thecytoplasm.

Poly-A

RNA transcript(pre-mRNA)

Intron

NUCLEUSCap

FORMATION OFINITIATION COMPLEX

After leaving thenucleus, mRNA attachesto the ribosome.

CYTOPLASM

Poly-A

Growingpolypeptide

Ribosomalsubunits

Aminoacyl-tRNAsynthetase

Aminoacid

tRNAAMINO ACID ACTIVATION

Each amino acidattaches to its proper tRNAwith the help of a specificenzyme and ATP.

Activatedamino acid

TRANSLATION

A succession of tRNAsadd their amino acids tothe polypeptide chainas the mRNA is movedthrough the ribosomeone codon at a time.(When completed, thepolypeptide is releasedfrom the ribosome.)

Anticodon

A A AU G G U U U A U G

U A CE A

Ribosome

Poly-A

Post-translation• The new polypeptide is now floating loose in the

cytoplasm if translated by a free ribosome. • Polypeptides fold spontaneously into their active

configuration, and they spontaneously join with other polypeptides to form the final proteins.

• Often translation is not sufficient to make a functional protein, polypeptide chains are modified after translation

• Sometimes other molecules are also attached to the polypeptides: sugars, lipids, phosphates, etc. All of these have special purposes for protein function.

Targeting Polypeptides to Specific Locations

• Completed proteins are targeted to specific sites in the cell

• Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER)– Free ribosomes mostly synthesize proteins that

function in the cytosol – Bound ribosomes make proteins of the endomembrane

system and proteins that are secreted from the cell

• Ribosomes are identical and can switch from free to bound

• Polypeptide synthesis always begins in the cytosol• Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to

attach to the ER• Polypeptides destined for the ER or for secretion are marked by a signal peptide • A signal-recognition particle (SRP) binds to the signal peptide• The SRP brings the signal peptide and its ribosome to the ER

Targeting Polypeptides to Specific Locations

Ribosomes

mRNASignalpeptide

Signal-recognitionparticle(SRP)

SRPreceptorprotein

CYTOSOL

ER LUMEN Translocationcomplex

Signalpeptideremoved

ERmembrane

Protein

Mutation Causes and Rate• The natural replication of DNA produces occasional errors.

DNA polymerase has an editing mechanism that decreases the rate, but it still exists

• Typically genes incur base substitutions about once in every 10,000 to 1,000,000 cells

• Since we have about 6 billion bases of DNA in each cell, virtually every cell in your body contains several mutations

• Mutations can be harmful, lethal, helpful, silent• However, most mutations are neutral: have no effect• Only mutations in cells that become sperm or eggs—are

passed on to future generations• Mutations in other body cells only cause trouble when they

cause cancer or related diseases

Mutagens• Mutagens are chemical or physical agents that interact

with DNA to cause mutations.• Physical agents include high-energy radiation like X-rays

and ultraviolet light • Chemical mutagens fall into several categories.

– Chemicals that are base analogues that may be substituted into DNA, but they pair incorrectly during DNA replication.

– Interference with DNA replication by inserting into DNA and distorting the double helix.

– Chemical changes in bases that change their pairing properties.• Tests are often used as a preliminary screen of chemicals

to identify those that may cause cancer• Most carcinogens are mutagenic and most mutagens are

carcinogenic.

Viral Mutagens• Scientists have recognized a number of tumor

viruses that cause cancer in various animals, including humans

• About 15% of human cancers are caused by viral infections that disrupt normal control of cell division

• All tumor viruses transform cells into cancer cells through the integration of viral nucleic acid into host cell DNA.

Point mutations• Point mutations involve alterations in the

structure or location of a single gene. Generally, only one or a few base pairs are involved.

• Point mutations can signficantly affect protein structure and function

• Point mutations may be caused by physical damage to the DNA from radiation or chemicals, or may occur spontaneously

• Point mutations are often caused by mutagens

Point Mutation• The change of a single nucleotide in the DNA’s template

strand leads to the production of an abnormal protein

In the DNA, themutant templatestrand has an A where the wild-type template has a T.

The mutant mRNA has a U instead of an A in one codon.

The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

Mutant hemoglobin DNAWild-type hemoglobin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

C T T C A T

G A A G U A

3 5 3 5

5 35 3

Types of Point Mutations

• Point mutations within a gene can be divided into two general categories– Base-pair substitutions - is the replacement of

one nucleotide and its partner with another pair of nucleotides

– Base-pair insertions or deletions - are additions or losses of nucleotide pairs in a gene

Base-Pair Substitutions• Silent - changes a codon but codes for the same amino acid• Missense - substitutions that change a codon for one amino acid into a

codon for a different amino acid• Nonsense -substitutions that change a codon for one amino acid into a

stop codonWild type

A U G A A G U U U G G C U A AmRNA 5Protein Met Lys Phe Gly Stop

Carboxyl endAmino end

A U G A A G U U U G G U U A A

Met Lys Phe Gly

Base-pair substitutionNo effect on amino acid sequence

U instead of C

A U G A A G U U U A G U U A A

Met Lys Phe Ser Stop

A U G U A G U U U G G C U A A

Met Stop

Missense A instead of G

NonsenseU instead of A

Insertions and Deletions– Are additions or losses of nucleotide pairs in a gene– May produce frameshift mutations that will change the reading

frame of the gene, and alter all codons downstream from the mutation.

mRNAProtein

Wild typeA U G A A G U U U G G C U A A

5Met Lys Phe Gly

Amino end Carboxyl end

Base-pair insertion or deletionFrameshift causing immediate nonsense

A U G U A A G U U U G G C U A

A U G A A G U U G G C U A A

A U G U U U G G C U A A

Met Stop

Met Lys Leu Ala

Met Phe Gly Stop

MissingA A G

Missing

Extra U

Frameshift causing extensive missense

Insertion or deletion of 3 nucleotides:no frameshift but extra or missing amino acid

DNA structure replication transcription translation

Health & Medicine

Replication, Transcription, & Translation

Replication, Transcription, and Translation

DNA PROCESSES Replication, Transcription and Translation.

Replication, transcription, translation and expression of...

DNA Replication, Transcription, Translation

DNA: Replication Transcription Translation

NAME Circle One: A1 A2 SCORE /16a. Replication,...

The Central Dogma of Molecular Biology replication...

CSI5126. Algorithms in bioinformatics - subtitle Central...

Dna Replication, Translation, And Transcription

Replication, Transcription, Translation

Review The mitochondrial genome: structure, transcription,.....

CENTRAL DOGMA: Replication Transcription Translation ·...

TRANSCRIPTION – TRANSLATION - GENETIC CODE AND...

DNA replication, transcription, the genetic code, and...

DNA Replication, Transcription and Translation